STUDY AND CHARACTERISTIC
OF ABRASIVE WEAR MECHANISMS
Ingrid KOVAŘÍKOVÁ, Beáta SZEWCZYKOVÁ, Pavel BLAŠKOVITŠ,
Erika HODÚLOVÁ, Emil LECHOVIČ
Authors:
Ing. Ingrid Kovaříková, PhD., Ing. Beáta Szewczyková, Prof. Ing. Pavel
Blaškoviš, DrSc., Ing. Erika Hodúlová, PhD., Ing. Emil Lechovič
Workplace:
Institute of Production Technologies, Faculty of Materials Science and
Technology, Slovak University of Technology
Address:
ul. Jána Bottu 23, 917 24 Trnava, Slovak Republic
E-mail:
ingrid.kovarikova@stuba.sk, beata.szewczykova@stuba.sk,
pavel.blaskovits@stuba.sk, erika.hodulova@stuba.sk, emil.lechovic@stuba.sk
Abstract
The main subjects of this paper are the tribotechnical system and abrasive wear.
Regarding the tribotechnical system essential information on structure, real contact
geometry, tribological loads, operating and loss variables are provided. Wear by abrasion is
form of wear caused by contact between a particle and solid material. Abrasive wear is the
loss of material by the passage of hard particles over a surface. Abrasion in particular is
rapid and severe forms of wear and can result in significant costs if not adequately
controlled. These differences extend to the practical consideration of materials selection for
wear resistance due to the different microscopic mechanisms of wear occurring in abrasion.
The questions are: where are abrasive wear likely to occur? When do these forms of wear
occur and how can they be recognized? What are the differences and similarities between
other types of wear?
Key words
tribology, tribotechnical system, wear mechanisms, abrasive wear, wear resistance
1
Tribology
Tribology is the science and technology of interacting surfaces in relative motion.
Tribology includes boundary-layer interactions both between solids and between solids and
liquids and/or gases. Tribology encompasses the entire field of friction and wear, including
lubrication (fig. 1) [1].
Tribology aims to optimize friction and wear for a particular application case. Apart from
fulfilling the required function, this means assuring high efficiency and sufficient reliability at
the lowest possible manufacturing, assembly, and maintenance costs.
Fig. 1 Expanded representation of a tribotechnical system (TTS) [2]
Tribotechnical system
General Description
Friction and wear occur within a tribotechnical system (TTS). To delimit a TTS, a system
envelope is appropriately placed around the components and materials directly involved in
friction and wear, thus virtually isolating these from the remaining components. The materials
and components involved in friction and wear are the elements of the TTS and are
characterized by their material and shape properties. A tribotechnical system is described by
the function to be fulfilled, the input variables (operating variables), the output variables, the
loss variables, and the structure (Fig. 1).
Structure
The elements involved, their properties, and the interactions between the elements
describe the structure of a TTS. The basic structure of all TTS consists of four elements: the
base body, counterbody, interfacial medium, and ambient medium (Fig. 1). Table 1 displays
some TTS with different elements. While the base body and counterbody are found in every
TTS, the interfacial medium and, in a vacuum, even the ambient medium can be absent.
2
EXAMPLES THE TRIBOTECHNICAL SYSTEMS ELEMENTS [2]
TTS
Base body
Counterbody
Table 1
Interfacial
Ambient medium
System type
-
Air
Closed
Press and shrink
joints
Shaft
Hub
Sliding bearing
Journal
Bearing bush
Oil
Air
Closed
Mechanical face
seal
Seal head
Seat
Liquid or gas
Air
Closed
Gear train
Pinion
Wheel
Gear oil
Air
Closed
Wheel/rail
Wheel
Rail
Moisture, dust,
grease
Air
Open
Excavator
bucket/excavated
material
Bucket
Excavated material
-
Air
Open
Turning tool
Cutting edge
Work piece
Cutting
lubricant
Air
Open
Tribologically relevant properties of elements of the tribotechnical system (TTS) [1].
1. Base body and counterpart
1.1 Geometric properties
• External dimensions
• Waviness
• Shape and position tolerances
• Surface roughness’s
1.2 Material properties
1.2.1 Bulk material
• Strength
• Modulus of el. Poisson’s ratio
• Hardness (macro, micro, and Martens hardness) • Residual stress
• Structure, texture, microstructure phases
• Chemical composition
(distribution, size, number type)
1.2.2 Near-surface zone
• Hardness (macro, micro, and Martens hardness) • Modulus of el. Poisson’s ratio
• Surface energy
• Residual stress
• Metallurgical structures, texture, microstructure • Boundary-layer thickness and structure
• Chemical composition
phases (distribution, size, number type)
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1.3 Physical variables
• Density
• Melting point
• Heat conductivity
• Spec. thermal capacity
• Coefficient of thermal expansion
• Hygroscopic properties
2. Interfacial medium (lubricant)
• For solid interfacial medium
• Aggregate state (solid, liquid, gaseous)
• For liquid interfacial medium
– Hardness – Viscosity depending on temperature, pressure, shear rate
– Grain size distribution – Consistency
– Grain shape – Wettability
– Grain quantity, grain number – Lubricant quantity and pressure
– Number of components, mixing ratio – Chemical composition
– Chemical composition – Mixing ratio of components
3. Ambient medium
• Aggregate state (solid, liquid, gaseous)
• Moisture
• Heat conductivity
• Ambient pressure
• Chemical composition
Wear
General
As soon as the base body and counterbody come into contact, i. e., when the lubrication
film thickness becomes too small or lubricant is unavailable, wear occurs. Wear is
a progressive loss of material from the surface of a solid, brought about by mechanical causes,
i. e., by contact and relative motion of a solid, fluid or gaseous counterbody (fig. 1). Signs of
wear are small detached wear particles, material removal from one friction body to the other,
and material and shape changes of the tribologically loaded material zone of one or both
friction partners.
Types and Mechanisms of Wear
Wear processes can be classified into different types according to the type of tribological
load and the materials involved, e.g., sliding wear, fretting wear, abrasive wear, and material
cavitation. Wear is caused by a number of mechanisms, the following four being especially
important:
• Surface fatigue
• Abrasion
• Adhesion
• Tribochemical reaction
4
Fig. 2 Basic wear mechanisms viewed microscopically (Fn normal force on apparent contact
surface, Ff friction force between base body and counterbody, Fn, as normal force on
asperity contact, ∆v relative velocity, HV Vickers hardness) [3]
Figure 2 presents a chart of the effective wear mechanisms. The wear mechanisms can
occur individually, successively or concomitantly. Surface fatigue manifests itself through
cracking, crack growth, and detachment of wear particles, brought about by alternating loads
in near-surface zones of the base body and counterbody.
In abrasion microcuttings, fatigue due to repeated ploughing, and fracture of the base
body caused by the counterbody’s hard asperities or by hard particles in the interfacial
medium lead to wear.
In adhesion, after possibly extant protective surface layers have been broken through,
atomic bonds (microwelds) form above all on the plastically deformed microcontacts between
the base body and counterbody. If the strength of the adhesive bonds is greater than that of the
softer friction partner, material eventually detaches from the deformed surface of the softer
friction partner and is transferred to the harder one. The transferred material can either remain
on the harder friction partner or detach, or even return.
In tribochemical reactions, friction-induced activation of loaded near-surface zones
causes elements of the base body and/or counterbody to react chemically with elements of the
lubricant or ambient medium. Compared with the base body and counterbody, the reaction
products exhibit changed properties and, after reaching a certain thickness, can be subject to
brittle chipping or even exhibit properties reducing friction and/or wear.
Apart from the types and mechanisms of wear, wear phenomena are also extremely
interesting for interpreting the result of wear (Table 2). These mean the changes of a body’s
surface layer resulting from wear and the type and shape of the wear particles accumulating.
Light or scanning electron microscope images can present this extremely clearly (1, 4).
5
TYPICAL WEAR PHENOMENA CAUSED BY THE MAIN WEAR MECHANISMS [1]
Table 2
Wear mechanism
Wear phenomenon
Adhesion
Scuffing or galling areas, holes, plastic shearing,
material transfer
Abrasion
Scratches, grooves, ripples
Surface fatigue
Cracks, pitting
Tribochemical reaction
Reaction products (layers, particles)
Abrasive Wear
Abrasive wear occurs whenever a solid object is loaded against particles of a material
that have equal or greater hardness. A common example of this problem is the wear of shovels
on earth-moving machinery. The extent of abrasive wear is far greater than may be realized.
Any material, even if the bulk of it is very soft, may cause abrasive wear if hard particles are
present. For example, an organic material, such as sugar cane, is associated with abrasive
wear of cane cutters and shredders because of the small fraction of silica present in the plant
fibers [5]. A major difficulty in the prevention and control of abrasive wear is that the term
‘abrasive wear’ does not precisely describe the wear mechanisms involved. There are, in fact,
almost always several different mechanisms of wear acting in concert, all of which have
different characteristics.
Mechanisms of Abrasive Wear
It was originally thought that abrasive wear by grits or hard asperities closely resembled
cutting by a series of machine tools or a file. However, microscopic examination has revealed
that the cutting process is only approximated by the sharpest of grits and many other more
indirect mechanisms are involved. The particles or grits may remove material by
microcutting, microfracture, pull-out of individual grains [6] or accelerated fatigue by
repeated deformations as illustrated in Figure 3.
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a) Cutting
b) Fracture
c) Fatigue by repeated ploughing
d) Grain pull-out
Fig. 3 Mechanisms of abrasive wear: microcutting, fracture, fatigue and grain pull-out [6]
The first mechanism illustrated in Figure 3a, cutting, represents the classic model where
a sharp grit or hard asperity cuts the softer surface. The material which is cut is removed as
wear debris. When the abraded material is brittle, e.g. ceramic, fracture of the worn surface
may occur (Figure 3b). In this instance wear debris is the result of crack convergence. When
a ductile material is abraded by a blunt grit then cutting is unlikely and the worn surface is
repeatedly deformed (Figure 3c). In this case wear debris is the result of metal fatigue. The
last mechanism illustrated (Figure 3d) represents grain detachment or grain pull-out. This
mechanism applies mainly to ceramics where the boundary between grains is relatively weak.
In this mechanism the entire grain is lost as wear debris.
Modes of Abrasive Wear
The way the grits pass over the worn surface determines the nature of abrasive wear. The
literature denotes two basic modes of abrasive wear:
· two-body and
· three-body abrasive wear.
Two-body abrasive wear is exemplified by the action of sand paper on a surface. Hard
asperities or rigidly held grits pass over the surface like a cutting tool. In three-body abrasive
wear the grits are free to roll as well as slide over the surface, since they are not held rigidly.
The two and three-body modes of abrasive wear are illustrated schematically in Figure 4.
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Two-body mode
Three-body mode
Fig. 4 Two and three-body modes of abrasive wear [7]
Until recently these two modes of abrasive wear were thought to be very similar,
however, some significant differences between them have been revealed [7]. It was found that
three-body abrasive wear is ten times slower than two-body wear since it has to compete with
other mechanisms such as adhesive wear [8]. Properties such as hardness of the “backing
wheel”, which forces the grits onto a particular surface, were found to be important for threebody but not for two-body abrasive wear. Two-body abrasive wear corresponds closely to the
‘cutting tool’ model of material removal whereas three-body abrasive wear involves slower
mechanisms of material removal, though very little is known about the mechanisms involved
[9]. It appears that the worn material is not removed by a series of scratches as is the case with
two-body abrasive wear. Instead, the worn surface displays a random topography suggesting
gradual removal of surface layers by the successive contact of grits [10].
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Conclusion
The mechanisms of abrasive wear is extremely interesting for interpreting the wear
because abrasive wear is very common type of wear in mining, agriculture, cement industry,
civil engineering, metallurgy.
Depends of the mechanisms we can prepare working surfaces with exact chemical
composition and heat treatment for example by hard facing technologies.
Acknowledgement
This work was supported by the Slovak research and Development Agency under the
contract No APVV-0057-07, VEGA 1/0381/08, APVT-20-011004.
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